diff --git a/lib/moxml/adapter/nokogiri.rb b/lib/moxml/adapter/nokogiri.rb
index 3c8f48b..b9beef7 100644
--- a/lib/moxml/adapter/nokogiri.rb
+++ b/lib/moxml/adapter/nokogiri.rb
@@ -46,17 +46,12 @@ def create_native_processing_instruction(target, content)
         end
 
         def create_native_declaration(version, encoding, standalone)
-          doc = ::Nokogiri::XML::Document.new
-          doc.create_internal_subset("xml", nil, nil)
-          declaration = ::Nokogiri::XML::ProcessingInstruction.new(doc, "xml", declaration_content(version, encoding, standalone))
-          declaration
-        end
-
-        def declaration_content(version, encoding, standalone)
-          content = "version=\"#{version}\""
-          content << " encoding=\"#{encoding}\"" if encoding
-          content << " standalone=\"#{standalone}\"" if standalone
-          content
+          decl = ::Nokogiri::XML::ProcessingInstruction.new(
+            ::Nokogiri::XML::Document.new,
+            "xml",
+            build_declaration_attrs(version, encoding, standalone)
+          )
+          decl
         end
 
         def set_namespace(element, ns)
@@ -227,6 +222,15 @@ def serialize(node, options = {})
             save_with: save_options,
           )
         end
+
+        private
+
+        def build_declaration_attrs(version, encoding, standalone)
+          attrs = { "version" => version }
+          attrs["encoding"] = encoding if encoding
+          attrs["standalone"] = standalone if standalone
+          attrs.map { |k, v| %{#{k}="#{v}"} }.join(" ")
+        end
       end
     end
   end
diff --git a/lib/moxml/adapter/oga.rb b/lib/moxml/adapter/oga.rb
index fba8149..7fcb8da 100644
--- a/lib/moxml/adapter/oga.rb
+++ b/lib/moxml/adapter/oga.rb
@@ -39,13 +39,10 @@ def create_native_processing_instruction(target, content)
         end
 
         def create_native_declaration(version, encoding, standalone)
-          doc = ::Oga::XML::Document.new
-          doc.xml_declaration = ::Oga::XML::XmlDeclaration.new(
-            version: version,
-            encoding: encoding,
-            standalone: standalone,
+          ::Oga::XML::ProcessingInstruction.new(
+            name: "xml",
+            text: build_declaration_attrs(version, encoding, standalone),
           )
-          doc
         end
 
         def create_native_namespace(element, prefix, uri)
diff --git a/lib/moxml/adapter/ox.rb b/lib/moxml/adapter/ox.rb
index 5c72a70..5807efa 100644
--- a/lib/moxml/adapter/ox.rb
+++ b/lib/moxml/adapter/ox.rb
@@ -24,6 +24,12 @@ def create_document
           ::Ox::Document.new
         end
 
+        def create_native_declaration(version, encoding, standalone)
+          inst = ::Ox::Instruction.new("xml")
+          inst.value = build_declaration_attrs(version, encoding, standalone)
+          inst
+        end
+
         def create_native_element(name)
           element = ::Ox::Element.new(name)
           element.instance_variable_set(:@attributes, {})
diff --git a/lib/moxml/declaration.rb b/lib/moxml/declaration.rb
index f39177d..beba1e8 100644
--- a/lib/moxml/declaration.rb
+++ b/lib/moxml/declaration.rb
@@ -1,12 +1,18 @@
 # lib/moxml/declaration.rb
 module Moxml
   class Declaration < Node
+    ALLOWED_VERSIONS = ["1.0", "1.1"].freeze
+    ALLOWED_STANDALONE = ["yes", "no"].freeze
+
     def version
       extract_attribute("version")
     end
 
     def version=(new_version)
-      update_content("version", new_version)
+      unless ALLOWED_VERSIONS.include?(new_version)
+        raise ValidationError, "Invalid XML version: #{new_version}"
+      end
+      set_attribute("version", new_version)
     end
 
     def encoding
@@ -14,7 +20,14 @@ def encoding
     end
 
     def encoding=(new_encoding)
-      update_content("encoding", new_encoding)
+      if new_encoding
+        begin
+          Encoding.find(new_encoding)
+        rescue ArgumentError
+          raise ValidationError, "Invalid encoding: #{new_encoding}"
+        end
+      end
+      set_attribute("encoding", new_encoding)
     end
 
     def standalone
@@ -22,7 +35,10 @@ def standalone
     end
 
     def standalone=(new_standalone)
-      update_content("standalone", new_standalone)
+      unless new_standalone.nil? || ALLOWED_STANDALONE.include?(new_standalone)
+        raise ValidationError, "Invalid standalone value: #{new_standalone}"
+      end
+      set_attribute("standalone", new_standalone)
     end
 
     def declaration?
@@ -37,19 +53,24 @@ def extract_attribute(name)
       match && match[1]
     end
 
-    def update_content(name, value)
-      content = @native.content || ""
+    def set_attribute(name, value)
+      attrs = current_attributes
       if value.nil?
-        content.gsub!(/\s*#{name}="[^"]*"/, "")
+        attrs.delete(name)
       else
-        if content.include?("#{name}=\"")
-          content.gsub!(/#{name}="[^"]*"/, "#{name}=\"#{value}\"")
-        else
-          content << " #{name}=\"#{value}\""
-        end
+        attrs[name] = value
       end
-      @native.content = content.strip
-      self
+      update_content(attrs)
+    end
+
+    def current_attributes
+      @native.content.to_s.scan(/(\w+)="([^"]*)"/).each_with_object({}) do |(name, value), hash|
+        hash[name] = value
+      end
+    end
+
+    def update_content(attrs)
+      @native.content = attrs.map { |k, v| %{#{k}="#{v}"} }.join(" ")
     end
   end
 end
diff --git a/spec/moxml/comment_spec.rb b/spec/moxml/comment_spec.rb
index 87658de..a8ec933 100644
--- a/spec/moxml/comment_spec.rb
+++ b/spec/moxml/comment_spec.rb
@@ -26,7 +26,7 @@
 
   describe "serialization" do
     it "wraps content in comment markers" do
-      expect(comment.to_xml(pretty: false)).to eq("<!-- test comment -->")
+      expect(comment.to_xml(pretty: false)).to eq("<!--test comment-->")
     end
 
     it "escapes double hyphens" do
@@ -38,7 +38,7 @@
 
     it "handles special characters" do
       comment.content = "< > & \" '"
-      expect(comment.to_xml(pretty: false)).to eq("<!-- < > & \" ' -->")
+      expect(comment.to_xml(pretty: false)).to eq("<!--< > & \" '-->")
     end
   end
 
@@ -47,7 +47,7 @@
 
     it "adds to element" do
       element.add_child(comment)
-      expect(element.to_xml).to include("<!-- test comment -->")
+      expect(element.to_xml).to include("<!--test comment-->")
     end
 
     it "removes from element" do